地震波初至旅行时计算策略

Strategies of seismic wave first break travel-time calculation

以波前扩展法为基础,提出了适合强速度纵横向变化的有限差分法地震波初至旅行时外推策略,即在计算时采用二阶精度的有限差分法对程函方程进行了近似计算。在没有出现对负数开平方的情况下只计算三个可能的体波,当出现已知条件不满足或算法不稳定(如对负数开平方)的情况时就计算可能的3个首波和两个散射波,两种情况下都取其最小值作为地震初至旅行时。采用二分法的旅行时查找方法,完成了初至旅行时算法的改进。复杂模型试算结果表明,该方法提高了初至旅行时计算的精度和稳定性。

A strategy of seismic wave first break travel-time calculation with finite difference method was proposed on the basis of wavefront spread method. The strategy can adapt to the strongly lateral and/or vertical velocity variation media. The first break travel-times were calculated with the second order finite difference approximation in solving the eikonal equation. Without the circumstances of extracting the square roots of the negative number, only three probable body waves were calculated. Whether the given conditions are not satisfied or the algorithm is not stabilized （extracting the square root of the negative number）, three probable body waves and two scattering waves must be calculated simultaneously. The minimization of travel-times was selected as the seismic wave first break travel-time in both cases listed above. The dichotomy was employed in looking for the location of wavefront point. Thus, the algorithm of first break travel-time calculation was improved. The method was applied successfully with a complicated model in the seismic wave first break travel-time calculation, which shows its high precision and stability.